Hydraulic fracturing has been utilized for may years to stimulate the production of oil, gas or other formation fluids from subterranean formations. In hydraulic fracturing, a suitable fluid is introduced into the down-hole formation by way of a well bore under conditions of flow rate and pressure which are at least sufficient to create or extend a fracture into a desired portion of the formation. Various fluids have been utilized in hydraulic fracturing, however, most fluids utilized today are aqueous-based liquids.
The presence of clay in an oil and gas producing formation poses a problem for production from wells completed in such formations. Ordinarily, such clays are inert in the formation and do not disrupt the flow of hydrocarbons. When disturbed, however, by aqueous-based fluids used in well stimulation, for example, clay particles can swell and reduce formation permeability.
Clay swelling problems in the past have been addressed by preflushing with slugs of salt-containing water and using inorganic salts in the aqueous stimulation fluids. Quite often the salt of choice has been potassium chloride (KCl) which converts the clay to a less swellable form by cation exchange with Na.sup.+ ions present on the clay surfaces. Other salts include calcium chloride, ammonium chloride, and the like, typically dissolved in an aqueous preflush and/or in the aqueous stimulation fluid used for the formation treatment.
Clays dispersed throughout oil-producing formations may be described as stacked platelets with a net positive charge associated with the four short dimensional sides and a net negative charge with the two long dimensional faces. It is generally believed that the concept of surface charge may be used to understand the mechanisms involved in swelling inhibition. Since the large negatively charged surface is exposed to the surrounding solution, it attracts cations from the solution.
In order to inhibit the swelling phenomenon, minimization of the hydratable surface area of the clay is necessary. One way that this may be accomplished is by flocculating and decreasing the surface charge density, or by increasing the ionic strength of the aqueous phase, or both. By allowing cations with small charge-to-surface-area ratios to associate with the particle, the effective strength of the negatively charged, double-face platelet layer surfaces will be diminished, allowing greater platelet-platelet interaction. Increasing the ionic strength of the solution will also have the same effect.
In the case of KCl, it is generally believed that the potential for clay swelling is shunted via a cation exchange of potassium ions for the more hydration-enticing native cations, for instance, sodium. It has been found that K.sup.+ is much better at creating electrostatic links between the negatively charged faces of the stacked clay platelets than the abundant Na.sup.+, therefore allowing less osmotic migration of water to occur between the platelets. While a lower concentration of K.sup.+ ions relative to Na.sup.+ ions is needed to flocculate clays, NH.sub.4.sup.+ ions have been shown to be even better or equal to K.sup.+ in creating electrostatic links and reducing osmotic migration of water.
While salts may be effective in protecting the formation, several problems are associated therewith: (1) the amount of material needed for preparing an effective fluid may be very high and it is often difficult to dissolve such solid components in the treating fluids in the quantities required; (2) in environmentally sensitive areas, there may be limits on the permissible amount of chloride; and (3) the presence of salts may interact with other additive components in the stimulation fluid, such as, for example, viscosifying agents, the hydration of which is inhibited by such salts. Accordingly, there is a need for a down-hole clay stabilizing composition which is inert to other down-hole fluid additives, low in chloride ion and therefore environmentally tolerable and which has greatly enhanced clay stabilizing effectiveness compared to KCl and other similar salts.
In U.S. Pat. No. 4,842,073 to Himes et al., there are disclosed formation control additives for utilization in stimulation fluids to minimize formation permeability damage as a result of formation contact by the stimulation fluid. The formation control additives disclosed include N-alkylpyridinium halides, N,N,N-trialkylphenolammonium halides, N,N,N-trialkylbenzylammonium halides, N,N,N-dialkylmorpholinium halides, and alkyl quaternary ammonium salts of two mole oligomers of epihalohydrin wherein the alkyl radical is selected from the group of methyl, ethyl, propyl and butyl radicals.
U.S. Pat. No. 4,526,693 to Son et al. discloses a clay-based aqueous drilling fluid containing dissolved inorganic salts for use in drilling through either or both shale and salt formations. The fluid is prepared by dissolving quantities of ammonium chloride and sodium chloride in the aqueous fluid prior to mixing of the clay and other additives.
U.S. Pat. No. 4,580,633 to Watkins et al. discloses a method for treating a formation containing finely divided particulate material to increase the flow of fluids through the formation by first injecting an organic silicon compound in a hydrocarbon carrier fluid, followed by an injection of steam containing a compound which contains ammoniacal nitrogen selected from the group consisting of ammonium hydroxide, ammonium salts of inorganic acids, ammonium salts of carboxylic acids, quaternary ammonium halides, amine or substituted amine hydrochlorides, derivatives of ammonium cyanate and water-soluble ammonia or ammonium ion precursors selected from the group of amides of carbamic acid and thiocarbamic acid, derivatives of such amides, tertiary carboxylic acid amides and their substituted and alkylated derivatives. A preferred nitrogen-containing compound is said to be urea.
In U.S. Pat. No. 3,412,019 to Hoover et al., there is disclosed a method for flocculating suspended matter in an aqueous media with polymers comprising repeating units derived from diallylamine and quaternary ammonium monomers containing groups condensed through a Michael addition reaction from a vinyl type activated double bond compound.
U.S. Pat. No. 3,797,574 to Feuerbacher et al. discloses a process for recovering petroleum from a subterranean formation by flooding with a surfactant which is inhibited from absorption by treating the formation with a pre-flush slug of a water soluble quaternary ammonium salt prior to injecting the surfactant.
Other references disclosing ammonium and/or alkali metal salts in various down-hole fluids include U.S. Pat. Nos. 3,089,874; 4,371,443; 3,882,029; 3,873,452; 3,707,192; 3,543,856; 2,713,033; and 4,572,296.